Open-loop diversity technique for systems employing multi-transmitter antennas

ABSTRACT

A method and apparatus for increasing the capacity of a system that use four transmit antennas and that employs conventional channel coding by performing space-time coding in a particular way. Each of two pairs of symbol sub-streams is space-time coded to form a respective pair of transmit-sequence chains, where at least one of the formed pairs of the transmit-sequence chains is a function of symbols of the respective symbol-sub-stream pair and not a function of the symbols of the other symbol-sub-stream pair. Four transmit sequences—two transmit sequences from each of the two pairs of symbol sub-streams—may be viewed as forming a transmission matrices B or B′ arranged as follows:  
                      T   1                     T   2                     T   3                     T   4                         Antenna         105   -   1             Antenna         105   -   2             Antenna         105   -   3             Antenna         105   -   4                [           b   1           b   1           -     b   2   *             -     b   2   *                 b   2           b   2           b   1   *           b   1   *               b   3           -     b   3             -     b   4   *             b   4   *               b   4           -     b   4             b   3   *           -     b   3   *             ]                     or     ,     
                       T   1                     T   2                     T   3                     T   4                       Antenna         105   -   1             Antenna         105   -   2             Antenna         105   -   3             Antenna         105   -   4                [           b   1           -     b   2   *           0       0             b   2           b   1   *         0       0           0       0         b   3           -     b   4   *               0       0         b   4           b   3   *           ]                  ,                 
 
     respectively, where b 1 , b 2 , b 3 , and b 4  are the symbols derived from a respective one of four symbol sub-streams, and b* 1 , b* 2 , b* 3 , and b* 4  are, respectively, the complex conjugate of the aforementioned symbols. The rows of the matrix represent the different antennas, while the columns represent different symbol periods.

BACKGROUND OF THE INVENTION

[0001] This invention relates to wireless communication systems, andmore particularly, to wireless communication systems using multipleantennas at the transmitter and one or more antennas at the receiver.

[0002] Wireless communication systems that use multiple antennas at thetransmitter are commonly referred to as multiple-input systems.Space-time coding can be used in multiple-input systems to reduce thepower needed to transmit information at a particular information datarate and still maintain a certain error rate.

[0003] Additionally, it is highly advantageous to employ channel codingin order to approach the maximum open loop capacity of a multiple-inputsystem. (An open loop system is one where the channel characteristicsare not fed back to the transmitter.) The state-of-the-art channelcodes, e.g., turbo codes, trellis codes and the like, are typicallyspatially one dimensional, i.e., they are designed for only a singletransmit antenna. A goal in the wireless communication industry is toemploy such codes in a multiple-input system.

SUMMARY OF THE INVENTION

[0004] One technique employing channel coding in a four-transmit-antennasystem uses a so-called decoupled space-time coding approach, describedin more detail in, U.S. patent application Ser. No. 09/752637, filed onDec. 29, 2000, entitled “Open-Loop Diversity Technique For SystemsEmploying Four Transmitter Antennas”.

[0005] The present inventors have recognized that it is possible tofurther increase the capacity of multiple-input systems that use fourtransmit antennas and that employ conventional channel coding byperforming the space-time coding in a particular way. Each of at leasttwo pairs of symbol sub-streams is space-time coded to form a respectivepair of transmit-sequence chains. At least one of the formed pairs ofthe transmit-sequence chains is a function of symbols of the respectivepair of symbol sub-streams and not a function of the symbols of theother pairs' symbol sub-streams. The symbol sub-streams are functions ofthe primitive data stream. The primitive data stream is channel coded,mapped into symbol space, and demultiplexed into the symbol sub-streams.

[0006] Illustratively, the symbol sub-steam pairs are space-time codedsuch that the first pair of transmit-sequence chains is a function ofthe symbols of a first symbol-sub-stream pair and not a function of thesymbols of a second symbol-sub-stream pair. The second pair oftransmit-sequence chains is a function of the symbols of the secondsymbol-sub-stream pair and not a function of the symbols of the firstsymbol-sub-stream pair.

[0007] In an illustrative embodiment of the invention, each transmitsequence of a particular transmit-sequence chain is formed from 1) asymbol of one of the symbol sub-streams of the respectivesymbol-sub-stream pair and 2) a complex conjugate of a symbol of theother symbol sub-stream of the pair. The symbol of 1) and symbol complexconjugate of 2) form a transmit sequence having a duration of foursymbol periods. Four transmit sequences—two transmit sequences formedfrom each of two symbol-sub-stream pairs—may be arranged as transmissionmatrix B. Each row of the matrix corresponds to a transmit antenna. Theelements of each row represent the symbols that are transmitted by therespective antenna in four symbol periods. The matrix B is arranged asfollows:   T₁  T₂  T₃  T₄ ${\begin{matrix}{Antenna} & {105 - 1} \\{Antenna} & {105 - 2} \\{Antenna} & {105 - 3} \\{Antenna} & {105 - 4}\end{matrix}\begin{bmatrix}b_{1} & b_{1} & {- b_{2}^{*}} & {- b_{2}^{*}} \\b_{2} & b_{2} & b_{1}^{*} & b_{1}^{*} \\b_{3} & {- b_{3}} & {- b_{4}^{*}} & b_{4}^{*} \\b_{4} & {- b_{4}} & b_{3}^{*} & {- b_{3}^{*}}\end{bmatrix}},$

[0008] where b₁, b₂, b₃, and b₄ are the symbols derived from arespective one of four symbol sub-streams, and b*₁, b*₂, b*₃, and b*₄are, respectively, the complex conjugate of the aforementioned symbols.The rows of the matrix represent the different transmit sequences, whilethe columns represent different symbol periods (T_(i), i=1 . . . 4).

[0009] In another embodiment of the invention, each transmit sequence ofa particular transmit-sequence chain is formed from 1) a symbol of oneof the symbol sub-streams of the respective symbol-sub-stream pair, and2) a complex conjugate of a symbol of the other symbol sub-stream of thepair. The symbol of 1) and symbol complex conjugate of 2) form atransmit sequence having a duration of four symbol periods, two of whichare blank. Four transmit sequences—two transmit sequences formed fromeach of the two symbol sub-stream pairs—may be arranged as atransmission matrix B′. Each row of the matrix corresponds to a transmitantenna. The elements of each row represent the symbols that are emittedby the respective antenna in four symbol periods. Matrix B′ is arrangedas follows:   T₁  T₂  T₃  T₄ ${\begin{matrix}{Antenna} & {105 - 1} \\{Antenna} & {105 - 2} \\{Antenna} & {105 - 3} \\{Antenna} & {105 - 4}\end{matrix}\begin{bmatrix}b_{1} & {- b_{2}^{*}} & 0 & 0 \\b_{2} & b_{1}^{*} & 0 & 0 \\0 & 0 & b_{3} & {- b_{4}^{*}} \\0 & 0 & b_{4} & b_{3}^{*}\end{bmatrix}}\quad$

[0010] Matrix B′ is orthogonal, that is B^(†)′B′ produces a matrix whosediagonal entries are non-zero and whose non-diagonal entries are zero,where B^(†)′ is the complex conjugate transpose of B′.

[0011] Note, matrix B can be obtained from matrix B′ by repeating eachof the non-zero elements of matrix B′ so that each symbol is in twosymbol periods. Furthermore to obtain B from B′, the signs of theelements of the second pair of transmit-sequence chains is changed whenthe symbol is repeated.

[0012] Advantageously, using the space-time coding approach of thepresent invention, the primitive data stream may be channel coded usingconventional channel coding techniques, e.g., turbo coding, prior tobeing space-time coded and the advantages of such channel coding may beexploited at the receiver.

BRIEF DESCRIPTION OF THE DRAWING

[0013]FIG. 1 illustrates a portion of a multiple-input wirelesscommunication system;

[0014]FIG. 2 illustrates in more detail a transmitter arranged inaccordance with the principles of the invention;

[0015]FIG. 3 illustrates in more detail a receiver arranged inaccordance with the principles of one embodiment of the invention; and

[0016]FIG. 4 illustrates a receiver arranged in accordance with theprinciples of another embodiment of the invention.

[0017] The figures are not drawn to scale and illustrate theinterconnectivity of the depicted systems and not necessarily theirspatial layout and physical dimensions.

DETAILED DESCRIPTION

[0018] As described above, a goal in the wireless communication industryis to employ channel codes in a multiple-input system. One techniqueemploying channel coding in a four-transmit-antenna system uses aso-called decoupled space-time coding approach. See for example, U.S.patent application Ser. No. 09/752637, filed on Dec. 29, 2000, entitled“Open-Loop Diversity Technique For Systems Employing Four TransmitterAntennas”, incorporated herein by this reference.

[0019]FIG. 1 illustrates multiple-input wireless communication system100 having four transmit antennas 105-1, 105-2, 105-3 and 105-4, and onereceive antenna 110. In system 100, primitive data stream 115—the datato be transmitted—is supplied to transmitter 120.

[0020]FIG. 2 shows, in more detail, exemplary transmitter 120 arrangedin accordance with the principles of the invention. Transmitter 120space-time codes each of the two pairs of symbol sub-streams to form arespective pair of transmit-sequence chains. The space-time coding issuch that a particular formed pair of transmit-sequence chains is afunction of symbols of the respective symbol sub-stream pair and not afunction of the symbols of the other symbol sub-stream pair.Illustratively, the symbol-sub-steam pairs are space-time coded suchthat the first pair of transmit-sequence chains is a function of thesymbols of the first symbol-sub-stream pair and not a function of thesymbols of a second symbol-sub-stream pair. The second pair oftransmit-sequence chains is a function of the symbols of the secondsymbol-sub-stream pair and not a function of the symbols of the firstsymbol-sub-stream pair. Symbols are a result of mapping the bits of the(channel-coded) primitive data streams into symbol space. The foursymbol sub-streams are processed so that they may be transmitted viafour transmit antennas.

[0021] Transmitter 120 processes primitive data stream 115. Primitivedata stream 115 is channel coded and mapped in encoder/mapper 135 toproduce symbol stream 125. Advantageously, encoder/mapper 135 may employconventional channel coding, such as turbo coding. Encoder/mapper 135may also perform the mapping of the bits to a discrete alphabet ormodulation symbol constellation after doing the actual coding.

[0022] The outputs of encoder/mapper 135 are referred to herein assymbols. Symbols are a result of mapping the bits of the channel-codedprimitive data stream into symbol space. (Note, that if the primitivedata stream is not channel-coded, that is transmitter 120 uses a mapperinstead of the encoder/mapper, then symbols are a result of mapping thebits of the uncoded primitive data stream into symbol space.) The timeduration of a symbol is referred to as a symbol period.

[0023] The channel coded and mapped symbol stream 125 is divided into aplurality of symbol sub-streams 137-1, 137-2, 137-3, and 137-4 typicallyby demultiplexing symbol stream 125 in demultiplexer 130 into theplurality of symbol sub-streams. (Typically, the number of symbolsub-streams equals the number of transmit antennas.)

[0024] The symbol sub-streams 137-1, 137-2, 137-3, and 137-4 aresupplied to space-time encoder 140 where every symbol period, space-timeencoder 140 processes the symbols of the symbol sub-streams to form apart of each of four transmit-sequence chains 142-1, 142-2, 142-3, and142-4. The transmit-sequence chains are composed of transmit sequences.Each transmit sequence spanning at least four symbol periods. Thesymbols are processed to develop their complex conjugate. Each transmitsequence of a particular transmit-sequence chain is formed from 1) asymbol of one of the symbol sub-streams of the respectivesymbol-sub-stream pair and 2) a complex conjugate of a symbol of theother symbol sub-stream of the respective symbol-sub-stream pair. Fourtransmit sequences—two transmit sequences formed from each of the twopairs of symbol sub-streams—that result from the operation of thespace-time encoder 140 can be represented as matrix B. Each row ofmatrix B corresponds to one of transmit antennas 105-1, 105-2, 105-3,and 105-4. More specifically, the elements of each row represent symbolsthat are emitted by the corresponding one of transmit antennas 105-1,105-2, 105-3, and 105-4 in four symbol periods. In such an embodiment ofthe invention, matrix B is: $\begin{matrix}{\quad {{{T1}\quad {T2}\quad {T3}\quad {T4}}{{\begin{matrix}{Antenna} & {105 - 1} \\{Antenna} & {105 - 2} \\{Antenna} & {105 - 3} \\{Antenna} & {105 - 4}\end{matrix}\begin{bmatrix}b_{1} & b_{1} & {- b_{2}^{*}} & {- b_{2}^{*}} \\b_{2} & b_{2} & b_{1}^{*} & b_{1}^{*} \\b_{3} & {- b_{3}} & {- b_{4}^{*}} & b_{4}^{*} \\b_{4} & {- b_{4}} & b_{3}^{*} & {- b_{3}^{*}}\end{bmatrix}},}}} & (1)\end{matrix}$

[0025] where, b₁ and b₂ are symbols of the first and second symbolsub-streams, respectively, of one symbol-sub-stream pair, b₃ and b₄ aresymbols of first and second symbol sub-streams, respectively, of theother symbol-sub-stream pair, and b*₁, b*₂, b*₃, and b*₄ are complexconjugates of b₁, b₂, b₃, and b₄, respectively. As indicated, each rowof matrix B represents a different transmit sequence, and each columnrepresents a different symbol period (T_(i), i=1 . . . 4).

[0026] The principles of the present invention can be used in a systemthat employs direct sequence spreading, such as, for example a codedivision multiple access (CDMA) system. In such a system, space-timeencoder 140 further multiplies each element of matrix B by a spreadingcode sequence represented by c, which spans 1 symbol period and containsN chips, where N is the spreading gain.

[0027] Each transmit-sequence chain 145-1, 145-2, 145-3, and 145-4developed by space-time encoder 140 is supplied as an input to arespective one of radio frequency (RF) units 145-1, 145-2, 145-3, and145-4. The radio frequency units perform all the necessary processing toconvert their respective transmit-sequence chains from baseband to radiofrequency modulated signals. The radio frequency modulated signaldeveloped by one of the RF units 145-1, 145-2, 145-3, and 145-4, issupplied to a respective one of the transmit antennas 105-1, 105-2,105-3, and 105-4 from which the signal is transmitted.

[0028] The channels between each pair of transmit and receive antennasare shown in FIG. 1. Each channel has its own channel characteristich_(i), where i=1, 2, 3, 4 and h_(i) represents the channelcharacteristic between the i^(th) transmit antenna and the receiveantenna. (The channel characteristic may also be referred to as achannel estimate or channel statistic.) Thus, the signal h_(i)S_(i) oneach channel is the transmitted signal from the channel's correspondingtransmit antenna as modified by the channel characteristic.

[0029] The transmitted signals S₁, S₂, S₃, and S₄, modified by therespective channel characteristics, arrive at the receive antenna 201.Thus, the received signal R_(S) at the receive antenna is asuperposition of the transmitted signals S₁, S₂, S₃, and S₄ as modifiedby the channel characteristics, plus noise η, making the receive antennasignal:

R _(S) =h ₁ S ₁ +h ₂ S ₂ +h ₃ S ₃ +h ₄ S ₄+η  (5)

[0030]FIG. 3 shows, in more detail, an exemplary embodiment of receiver200 arranged in accordance with the principles of the invention. Antenna201 supplies an electrical version of the received signal to RF unit203. RF unit 203 converts the radio frequency signal supplied to it byantenna 201 to a baseband version thereof.

[0031] Demultiplexer 202 divides the received baseband signal into fourreceived symbol sub-streams 204-1, 204-2, 204-3, and 204-4, with thesymbols received in one of four consecutive symbol periods allocated toa different received symbol sub-stream. Demultiplexer 202 provides eachof the received symbol sub-streams to a respective one of thecorrelators 205-1, 205-2, 205-3, and 205-4.

[0032] If the system uses direct sequence spreading, such as a codedivision multiple access (CDMA) system, each correlator 205 is suppliedwith an orthogonal spreading code sequence represented as a horizontalvector c. The orthogonal spreading code sequence c spans 1 symbol periodand contains N chips, where N is the spreading gain. Thus, correlators205 perform despreading, which is the inverse of the spreading performedin space-time encoder 140. Each correlator supplies a despread symbolsub-stream to selective conjugator 206. (If correlators 205 are notpresent, demultiplexer 202 provides the received symbol sub-streamsdirectly to respective inputs of selective conjugator 206.)

[0033] Selective conjugator 206 determines the complex conjugate of anyof the outputs d′_(i) of correlators 205 that is being sought in thesystem of linear equations that describes the input to matrix multiplier207. This system of linear equations is generally represented, when thechannel is a flat-faded channel, as d=Hb+noise. d is the vertical vectorthat is the output of selective conjugator 206, b is a vertical vectorformed from b₁, b₂, b₃, and b₄, and H is a matrix that is a function ofchannel characteristics h₁, h₂, h₃, and h₄ and matrix B. Note, H is thematrix derived from the channel characteristics where H maps a) thesymbols of the symbol sub-streams prior to space-time coding to b) thereceived signals after the selective conjugator. H is the matrix of thechannel characteristics if the space-time coding performed by space-timeencoder 140 of FIG. 2 is considered part of the operation of thechannel.

[0034] For example, when, as described hereinabove, matrix B is shown asmatrix (1), then $\begin{matrix}{{d = {\begin{bmatrix}d_{1} \\d_{2} \\d_{3}^{*} \\d_{4}^{*}\end{bmatrix} = {{\begin{bmatrix}h_{1} & h_{2} & h_{3} & h_{4} \\h_{1} & h_{2} & {- h_{3}} & {- h_{4}} \\h_{2}^{*} & {- h_{1}^{*}} & h_{4}^{*} & {- h_{3}^{*}} \\h_{2}^{*} & {- h_{1}^{*}} & {- h_{4}^{*}} & h_{3}^{*}\end{bmatrix}\begin{bmatrix}b_{1} \\b_{2} \\b_{3} \\b_{4}\end{bmatrix}} + \begin{bmatrix}\eta_{1} \\\eta_{2} \\\eta_{3} \\\eta_{4}\end{bmatrix}}}},} & (6)\end{matrix}$

[0035] where d*₃ and d*₄ are the complex conjugates of d₃ and d₄,respectively.

[0036] Matrix multiplier 207 operates on the received vertical vector dto produce four match-filtered outputs. To this end, matrix multiplier207 also receives, or derives, a 4×4 matrix H^(†), where † denotescomplex conjugate transpose, also known as Hermitian transpose. As notedabove, H, is the following matrix $\begin{matrix}\begin{bmatrix}h_{1} & h_{2} & h_{3} & h_{4} \\h_{1} & h_{2} & {- h_{3}} & {- h_{4}} \\h_{2}^{*} & {- h_{1}^{*}} & h_{4}^{*} & {- h_{3}^{*}} \\h_{2}^{*} & {- h_{1}^{*}} & {- h_{4}^{*}} & h_{3}^{*}\end{bmatrix} & (7)\end{matrix}$

[0037] thus H^(†) is $\begin{matrix}{\begin{bmatrix}h_{1}^{*} & h_{1}^{*} & h_{2} & h_{2} \\h_{2}^{*} & h_{2}^{*} & {- h_{1}} & {- h_{1}} \\h_{3}^{*} & {- h_{3}^{*}} & h_{4} & {- h_{4}} \\h_{4}^{*} & {- h_{4}^{*}} & {- h_{3}} & h_{3}\end{bmatrix},} & (8)\end{matrix}$

[0038] where, as described above, h_(i) is the complex channelcharacteristic of the channel from the i^(th) transmit antenna to thereceiver antenna if the space-time coding performed by space-timeencoder 140 of FIG. 2 is considered part of the operation of thischannel and assuming all the channels are flat-faded. The matrix H^(†)multiplies from the left the 4×1 vertical vector d formed by the outputsof correlators 205 to produce a new 4×1 vertical vector f, i.e.,f=H^(†)d.

[0039] Note that H is orthogonal, that is H^(†)H, produces a matrixwhose diagonal entries are non-zero and whose non-diagonal entries arezero. $\begin{matrix}{{H^{\dagger}H} = {\begin{bmatrix}{2\left( {{h_{1}}^{2} + {h_{2}}^{2}} \right)} & 0 & 0 & 0 \\0 & {2\left( {{h_{1}}^{2} + {h_{2}}^{2}} \right)} & 0 & 0 \\0 & 0 & {2\left( {{h_{3}}^{2} + {h_{4}}^{2}} \right)} & 0 \\0 & 0 & 0 & {2\left( {{h_{3}}^{2} + {h_{4}}^{2}} \right)}\end{bmatrix}.}} & (9)\end{matrix}$

[0040] The results of matrix multiplier 207 (i.e. the vertical vector f)are then supplied as an input to multiplexer (MUX) 213 which interleavesthem in the inverse pattern of DEMUX 130 to reconstruct the symbolstream, forming reconstructed symbol stream 225.

[0041] Reconstructed symbol stream 225 is then decoded by decoder 211 toproduce reconstructed primitive data stream 215. The decoding performedby decoder 211 is advantageously the inverse of that performed byencoder/mapper 135. Decoder 221 may also perform the de-mapping. (Notethat decoding may be eliminated entirely if coding was not performed byencoder/mapper 135, shown in FIG. 2.) Note that the decisionfunctionality that is part of the digital demodulation, that is processthat selects the closest constellation point (i.e. the de-mappingprocess), may be in decoders 211 or in a separate unit whosefunctionality is performed either before or after the functionality ofmultiplexer 213. Furthermore, the particular algorithm used to achievethe decision functionality is at discretion of the implementor.

[0042] In another embodiment of the invention, each transmit sequence ofa particular transmit-sequence chain is a formed from 1) a symbol of oneof the symbol sub-streams of the respective symbol-sub-stream pair, 2) acomplex conjugate of a symbol of the other symbol sub-stream of therespective symbol-sub-stream pair. The symbol of 1) and symbol complexconjugate of 2) form a transmit sequence having a duration of foursymbol periods, two of which are blank. Four transmit sequences—twotransmit sequences formed from each of the two pairs of symbolsub-streams —may be arranged as a transmission matrix B′. Each row ofmatrix B′ corresponds to a transmit antenna, and the elements of eachrow represent the symbols that are emitted by the corresponding antennain four symbol periods. The matrix B′ is arranged as follows:$\begin{matrix}{\quad {{T_{1}\quad T_{2}\quad T_{3}\quad T_{4}}{{\begin{matrix}{Antenna} & {105 - 1} \\{Antenna} & {105 - 2} \\{Antenna} & {105 - 3} \\{Antenna} & {105 - 4}\end{matrix}\begin{bmatrix}b_{1} & {- b_{2}^{*}} & 0 & 0 \\b_{2} & b_{1}^{*} & 0 & 0 \\0 & 0 & b_{3} & {- b_{4}^{*}} \\0 & 0 & b_{4} & b_{3}^{*}\end{bmatrix}}\quad,}}} & (10)\end{matrix}$

[0043] where, b₁ and b₂ are symbols of first and second symbolsub-streams, respectively, of one of the symbol-sub-stream pairs, b₃ andb₄ are symbols of first and second symbol sub-streams, respectively, ofanother of the symbol-sub-stream pairs, and b*₁, b*₂, b*₃, and b*₄ arecomplex conjugates of b₁, b₂, b₃, and b₄, respectively. As indicated,the rows of the matrix represent the different transmit sequences, whilethe columns represent different symbol periods (T_(i), i=1 . . . 4),with each column representing one symbol period.

[0044] The matrix B′ is orthogonal, that is B^(†)′B′ produces a matrixwhose diagonal entries are non-zero and whose non-diagonal entries arezero, where B^(†)′ is the complex conjugate transpose of matrix B′.Thus, $\begin{matrix}{{B^{\dagger\prime} = \begin{bmatrix}b_{1}^{*} & b_{2}^{*} & 0 & 0 \\{- b_{2}} & {- b_{1}} & 0 & 0 \\0 & 0 & b_{3}^{*} & b_{4}^{*} \\0 & 0 & {- b_{4}} & b_{3}\end{bmatrix}}\quad,{and}} & (11) \\{{B^{\dagger}B} = \begin{bmatrix}\left( {{b_{1}}^{2} + {b_{2}}^{2}} \right) & 0 & 0 & 0 \\0 & \left( {{b_{1}}^{2} + {b_{2}}^{2}} \right) & 0 & 0 \\0 & 0 & \left( {{b_{3}}^{2} + {b_{4}}^{2}} \right) & 0 \\0 & 0 & 0 & \left( {{b_{3}}^{2} + {b_{4}}^{2}} \right)\end{bmatrix}} & (12)\end{matrix}$

[0045] Similarly to the above-described embodiment, this embodiment canalso be used in a system that employs direct sequence spreading. Asdescribed above, space-time encoder 140 multiplies each element ofmatrix B′ by a spreading code sequence represented by c, which spans 1symbol period and contains N chips, where N is the spreading gain. Theresult is then supplied to frequency (RF) units 145-1, 145-2, 145-3, and145-4 that perform all the necessary processing to convert theirrespective transmit-sequence chains from baseband to radio frequencymodulated signals. The radio frequency modulated signal are thentransmitted over a respective one of the transmit antennas 105-1, 105-2,105-3, and 105-4.

[0046] The transmitted signals modified by the respective channelcharacteristics, arrive at the receive antenna 201 of receiver 400. FIG.4 shows an exemplary embodiment of receiver 400 arranged in accordancewith the principles of the invention. In this embodiment of theinvention, receiver 400 can be used instead of receiver 200 in system100 of FIG. 1. In receiver 400, demultiplexer 402 divides the receivedsignal into two received symbol sub-streams 404-1 and 404-2, as opposedto the four received symbol sub-streams of the above describedembodiment. Each of the received symbol sub-streams 404-1 and 404-2 isapplied one of two correlators 405-1 and 405-2, that produce a pair ofsymbol sub-streams in two symbol periods and another pair of symbolsub-streams in the subsequent two symbol periods. The symbol sub-streamsare further processed, either jointly or individually, to ultimatelydevelop a reconstructed version of the primitive data stream.

[0047] Particularly, as in the above embodiment, antenna 201 suppliesthe received radio frequency signal to RF unit 203. RF unit 203 convertsthe radio frequency signal to a baseband version thereof.

[0048] Demultiplexer 402 divides the received baseband signal into tworeceived encoded symbol sub-streams 404-1 and 404-2, with the symbolsreceived in one of two consecutive symbol periods being part of adifferent received encoded symbol sub-stream. Demultiplexer 402 suppliesone of the received encoded symbol sub-streams to one of the correlators405-1 and 405-2, respectively.

[0049] As described above, correlators 405-1 and 405-2 performdespreading. Despreading is the inverse of the spreading performed inspace-time encoder 140. The correlators 405-1 and 405-2 supply thedespread symbol sub-streams to selective conjugator 406. (If correlators405 are not present, demultiplexer 402 supplies the received encodedsymbol sub-streams directly to selective conjugator 406.)

[0050] Selective conjugator 406 determines the complex conjugate of anyof the outputs d′_(i) of correlators 405 that is being sought in thesystem of linear equations that describes the input to matrix multiplier407. When the channel is a flat-faded channel, this system of linearequations is generally represented as d₁₂=H₁₂b₁₂+noise in a certain pairof consecutive symbol periods, and d₃₄=H₃₄b₃₄+noise in a subsequent pairof consecutive symbol periods. d₁₂ is the vertical vector that is theoutput of selective conjugator 406 in the former pair of consecutivesymbol periods, and d₃₄ is the vertical vector that is the output ofselective conjugator 406 in the latter pair of consecutive symbolperiods. b₁₂ is a vertical vector formed from b₁ and b₂, and b₃₄ is avertical vector formed from b₃ and b₄. Matrix H₁₂ is a function ofchannel characteristics h₁, and h₂ and matrix B′. Matrix H₃₄ is afunction of channel characteristics h₃, and h₄ and matrix B′. Note, H₁₂and H₃₄ are the matrixes derived from the channel characteristics. H₁₂maps a) the symbols of the first symbol sub-stream pair prior tospace-time coding to b) the received signals after the selectiveconjugator. H₃₄ maps a) the symbols of the second symbol sub-stream pairprior to space-time coding to b) the received signals after theselective conjugator. H₁₂ and H₃₄ are the matrices of the channelcharacteristics if the space-time coding performed by space-time encoderof the transmitter is considered part of the operation of the channel.

[0051] For example, if matrix B′ is arranged in the manner describedhereinabove as matrix (13), then $\begin{matrix}{d_{12} = {\begin{bmatrix}d_{1} \\d_{2}^{*}\end{bmatrix} = {{\begin{bmatrix}h_{1} & h_{2} \\h_{2}^{*} & {- h_{1}^{*}}\end{bmatrix}\begin{bmatrix}b_{1} \\b_{2}\end{bmatrix}} + {\begin{bmatrix}\eta_{1} \\\eta_{2}\end{bmatrix}\quad {and}}}}} & (14) \\{d_{34} = {\begin{bmatrix}d_{3} \\d_{4}^{*}\end{bmatrix} = {{\begin{bmatrix}h_{3} & h_{4} \\h_{4}^{*} & {- h_{3}^{*}}\end{bmatrix}\begin{bmatrix}b_{3} \\b_{4}\end{bmatrix}} + \begin{bmatrix}\eta_{3} \\\eta_{4}\end{bmatrix}}}} & (15)\end{matrix}$

[0052] where d*₂ and d*₄ are the complex conjugates of d₂ and d₄,respectively.

[0053] Matrix multiplier 407 operates on the received vertical vectorsd₁₂ and d₃₄ to produce two match-filtered outputs. To this end, matrixmultiplier 407 also receives, or derives, two 2×2 matrixes H₁₂ ^(†) andH₃₄ ^(†), where † denotes complex conjugate transpose, also known asHermitian transpose. As noted above, H₁₂ and H₃₄, are the followingmatrices: $\begin{matrix}{{H_{12} = \begin{bmatrix}h_{1} & h_{2} \\h_{2}^{*} & {- h_{1}^{*}}\end{bmatrix}},{and}} & (16) \\{{H_{34} = {\begin{bmatrix}h_{3} & h_{4} \\h_{4}^{*} & {- h_{3}^{*}}\end{bmatrix}.\quad {Thus}}},} & (17) \\{{H_{12^{\dagger}} = \begin{bmatrix}h_{1}^{*} & h_{2} \\h_{2}^{*} & {- h_{1}}\end{bmatrix}},{and}} & (18) \\{{H_{34^{\dagger}} = \begin{bmatrix}h_{3}^{*} & h_{4} \\h_{4}^{*} & {- h_{3}}\end{bmatrix}},} & (19)\end{matrix}$

[0054] where, as described above, h_(i) is the complex channelcharacteristic of the channel from the i^(th) transmit antenna to thereceiver antenna if the space-time coding performed by space-timeencoder 140 of FIG. 2 is considered part of the operation of the channeland assuming all the channels are flat-faded channels. The matrixes H₁₂^(†) and H₃₄ ^(†) multiply from the left the 2×1 vertical vector d₁₂ andd₃₄, respectively, formed by the outputs of correlators 405-1 and 405-2to produce two new 2×1 vertical vectors f₁₂ and f₃₄, i.e., f₁₂=H₁₂^(†)d₁₂, and f₃₄=H₃₄ ^(†)d₃₄.

[0055] Note that H₁₂ is orthogonal, that is $\begin{matrix}{{H_{12^{\dagger}}H_{12}} = \begin{bmatrix}\left( {{h_{1}}^{2} + {h_{2}}^{2}} \right) & 0 \\0 & \left( {{h_{1}}^{2} + {h_{2}}^{2}} \right)\end{bmatrix}} & (20)\end{matrix}$

[0056] and that H₃₄ is orthogonal, that is $\begin{matrix}{{H_{34^{\dagger}}H_{34}} = {\begin{bmatrix}\left( {{h_{3}}^{2} + {h_{4}}^{2}} \right) & 0 \\0 & \left( {{h_{3}}^{2} + {h_{4}}^{2}} \right)\end{bmatrix}.}} & (21)\end{matrix}$

[0057] The results of matrix multiplier 407 are then supplied as aninput to multiplexer (MUX) 413 (FIG. 3) which interleaves them in theinverse pattern of DEMUX 130 to reconstruct the symbol stream, formingreconstructed symbol stream 225. As described above, reconstructedsymbol stream 225 is then decoded by decoder 211 to producereconstructed primitive data stream 215.

[0058] Although the present embodiment is described with receiver 400that separately obtains each pair of symbol sub-streams, in alternativeembodiments the receiver can be similar to receiver 200 with$\begin{matrix}{d = {\begin{bmatrix}d_{1} \\d_{2}^{*} \\d_{3} \\d_{4}^{*}\end{bmatrix} = {{\begin{bmatrix}h_{1} & {- h_{2}^{*}} & 0 & 0 \\h_{2} & h_{1}^{*} & 0 & 0 \\0 & 0 & h_{3} & {- h_{4}^{*}} \\0 & 0 & h_{4} & h_{3}^{*}\end{bmatrix}\begin{bmatrix}b_{1} \\b_{2} \\b_{3} \\b_{4}\end{bmatrix}} + {\begin{bmatrix}\eta_{1} \\\eta_{2} \\\eta_{3} \\\eta_{4}\end{bmatrix}\quad {and}}}}} & (22) \\{H^{\dagger} = {\begin{bmatrix}h_{1}^{*} & h_{2} & 0 & 0 \\{- h_{2}^{*}} & h_{1} & 0 & 0 \\0 & 0 & h_{3}^{*} & h_{4} \\0 & 0 & {- h_{4}^{*}} & h_{3}\end{bmatrix}.}} & (23)\end{matrix}$

[0059] In the latter-described illustrative embodiment using matrix B′,in some symbol periods nothing is transmitted on some of the transmitantennas. For example, in the first symbol period there are symbolstransmitted on antennas 105-1 and 105-2 and there are no symbolstransmitted on antennas 105-3 and 105-4. Thus, sometimes there is zeropower on some of the transmit antennas. This may produce a distortedpower spectrum envelope with power outside the power spectrum envelopeallowed to the service provider of system 100 by the FCC. The formerdescribed illustrative embodiment, using matrix B, does not raise thisconcern.

[0060] When total power of the transmitter remains the same when eithermatrix B′ or B is used, because only two antennas are used at particulartime to transmit the elements of matrix B′, then the power on anyparticular one of these antenna is twice power on a particular antennatransmitting the elements of matrix B.

[0061] Matrixes B and B′ of the two above described illustrativeembodiments are related in that the elements of matrix B can be obtainedfrom the elements of the matrix B′ by repeating each of the non-zeroelements of matrix B′ so that each symbol is in two symbol periods, and,furthermore, the signs of the elements of the second pair oftransmit-sequence chains of is changed when the symbol is repeated.Thus, the first non-zero element of each of the transmission sequencesthat form matrix B′ is the first and second elements of the transmissionsequences that form B, and the second non-zero element of each of thetransmission sequences that form B′ is the third and fourth elements ofthe transmission sequences that form B.

[0062] Advantageously, using the space-time coding approach of thepresent invention, the primitive data stream may be channel coded usingconventional channel coding techniques, e.g., turbo coding, and theadvantages of such channel coding may be exploited at the receiver.

[0063] The foregoing is merely illustrative and various alternativeswill now be discussed. For example, the matrixes of the illustrativeembodiments are illustrated using b₁, b₂, b₃, and b₄. In alternativeembodiments any transmit sequences can be used where each of at leasttwo pairs of symbol sub-streams is space-time coded to form a respectivepair of transmit-sequence chains. The space-time coding is such that atleast one of the formed pairs of the transmit-sequence chains is afunction of symbols of the respective pair of symbol sub-streams and nota function of the symbols of the other pairs of the symbol sub-streams.

[0064] In the illustrative embodiment the system is in use in anenvironment having flat-faded channels. Those of ordinary skill in theart of non-flat-faded channels will be able to apply the techniques ofthe invention for use with non-flat-faded channels.

[0065] In the illustrative embodiment encoder/mapper 135 performs boththe channel coding and the mapping. In alternative embodiments, thefunctionality of either the channel coding or the mapping can be eitheromitted or performed either earlier or latter in the process.Additionally, although the encoder/mapper is shown as one functionalblock, it can be implemented as either one or as separate functionalblocks, in either hardware or software or a combination of hardware andsoftware.

[0066] In the illustrative embodiment the system is a multiple-inputsystem. In the alternative embodiments the system can be amultiple-input, multiple-output; where a multiple-input, multiple-outputsystem is one that has multiple antennas at the transmitter and multipleantennas at the receiver. In the latter case the symbols arereconstructed for each of the received symbol sub-streams developed ateach receive antenna in the manner described hereinabove. They may thenbe combined to develop an improved estimate of the original symbol. Suchcombination may be achieved, for example, by averaging values for eachcorresponding symbol.

[0067] The illustrative embodiments use RF antennas. In alternativeembodiments any form of antennas may be employed, e.g., a light source.Furthermore, although radio frequency units are shown in theillustrative embodiments, in other embodiments of the invention, e.g.,those using light for communicating the transmitted signal, differentmodulators and demodulators may be employed.

[0068] In the illustrative embodiment, encoder/mapper 135 processes theprimitive data stream and the resulting symbol stream is divided into aplurality of symbol sub-streams. In alternative embodiments, theprimitive data stream may be first divided into a plurality of datasub-streams each of which would then be processed by a respective one ofa plurality of encoder/mappers and the plurality of resulting symbolsub-streams supplied to space time encoder 140. In the latter case thechannel coding of each data sub-stream may be independent of the channelcoding of the other data sub-streams.

[0069] In the illustrative embodiments that uses matrix B′ nothing istransmitted on some of the transmit antennas in some symbol periods. Inalternative embodiments, known symbols may be transmitted instead of nottransmitting anything on those antennas. These known symbols will thenbe subtracted out.

[0070] The transmitter and receiver embodying the principles of thepresent invention can be used in various parts of a wirelesscommunication system in addition, or instead of, the ones shown in theillustrative embodiments. For example, the transmitter can be part of abase station and the receiver part a mobile terminal, and/or vice versa,i.e. the transmitter can be part of the mobile terminal and the receiverpart of the base station. Alternatively, the transmitter can be part ofa wireless hub of a wireless local area network and the receiver part aterminal of a wireless local area network, such as a laptop, and/or viceversa. Or, each of the transmitter and receiver can be part of a fixedwireless network, for example the transmitter and receiver can be partof a fixed wireless system set up for communication between twobuildings.

[0071] The block diagrams presented in the illustrative embodimentsrepresent conceptual views of illustrative circuitry embodying theprinciples of the invention. Any of the functionally of the illustrativecircuitry can be implemented as either a single circuit or as multiplecircuits. The functionality of multiple illustrative circuitry can alsobe implemented as a single circuit. Additionally, one or more of thefunctionally of the circuitry represented by the block diagrams may beimplemented in software by one skilled in the art with access to theabove descriptions of such functionally.

[0072] Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

[0073] Thus, while the invention has been described with reference to apreferred embodiment, it will be understood by those skilled in the arthaving reference to the specification and drawings that variousmodifications and alternatives are possible therein without departingfrom the spirit and scope of the invention.

We claim:
 1. A method for use in a system adapted to transmit at least four series of transmit sequences over at least four transmit antennas, the method comprising the step of: space-time coding at least two pairs of symbol sub-streams, each of the pairs of symbol streams being space-time coded to form a respective pair of the transmit-sequence chains, the space-time coding being such that at least one of the formed pairs of the transmit-sequence chains is a function of symbols of the respective pair of symbol sub-streams and not a function of the symbols of the other pairs of the symbol sub-streams.
 2. The invention of claim 1, wherein each transmit sequence has a duration of four symbol periods; each transmit sequence of a particular transmit-sequence chains is a function of 1) a symbol of one of the symbol sub-streams of the respective symbol-sub-stream pair and 2) a complex conjugate of a symbol of the other symbol sub-stream of the respective symbol-sub-stream pair; and portions of the at least four transmit-sequence chains are representable by a matrix where: each row of the matrix represents one transmit sequence of a respective different one of the transmit-sequence chains, and each column of the matrix represents one symbol period.
 3. The invention of claim 2, wherein the matrix is orthogonal.
 4. The invention of claim 1, wherein portions of the at least four transmit-sequence chains are representable by a matrix where: each row of the matrix represents one transmit sequence of a respective different one of the transmit-sequence chains; each column of the matrix represents one symbol period; and the matrix is $\begin{bmatrix} b_{1} & b_{1} & {- b_{2}^{*}} & {- b_{2}^{*}} \\ b_{2} & b_{2} & b_{1}^{*} & b_{1}^{*} \\ b_{3} & {- b_{3}} & {- b_{4}^{*}} & b_{4}^{*} \\ b_{4} & {- b_{4}} & b_{3}^{*} & {- b_{3}^{*}} \end{bmatrix},$

where: b₁ and b₂ are symbols of first and second symbol sub-streams, respectively, of one of the symbol-sub-stream pairs, b₃ and b₄ are symbols of first and second symbol sub-streams, respectively, of another of the symbol-sub-stream pairs, and b*₁, b*₂, b*₃, and b*₄ are complex conjugates of b₁, b₂, b₃, and b₄, respectively.
 5. The invention of claim 1, wherein portions of the at least four transmit-sequence chains are representable by a matrix where: each row of the matrix represents one transmit sequence of a respective different one of the transmit-sequence chains; each column of the matrix represents one symbol period; and the matrix is $\begin{bmatrix} b_{1} & {- b_{2}^{*}} & 0 & 0 \\ b_{2} & b_{1}^{*} & 0 & 0 \\ 0 & 0 & b_{3} & {- b_{4}^{*}} \\ 0 & 0 & b_{4} & b_{3}^{*} \end{bmatrix},$

where: b₁ and b₂ are symbols of first and second symbol sub-streams, respectively, of one of the symbol-sub-stream pairs, b₃ and b₄ are symbols of first and second symbol sub-streams, respectively, of another of the symbol-sub-stream pairs, and b*₁, b*₂, b*₃, and b*₄ are complex conjugates of b₁, b₂, b₃, and b₄, respectively.
 6. The invention of claim 1, wherein the space-time coding step comprises the steps of: space-time coding a first pair of symbol sub-streams to form a first pair of transmit-sequence chains, the first pair of transmit-sequence chains being a function of the symbols of the first symbol-sub-stream pair and not a function of the symbols of a second symbol-sub-stream pair; and space-time coding the second pair of symbol sub-streams to form a second of transmit-sequence chains, the second pair of transmit-sequence chains being a function of the symbols of the second symbol-sub-stream pair and not a function of the symbols of the first symbol-sub-stream pair.
 7. The invention of claim 1, further comprising the step of transmitting the at least four transmit-sequence chains on a respective one of the transmit antennas.
 8. The invention of claim 1, further comprising the step of spreading at least a plurality of symbols of the transmit-sequence chains using a spreading code.
 9. The invention of claim 1, further comprising the steps of: channel coding each of at least four data sub-streams using a channel code; and mapping each of the channel-coded primitive data stream into symbol-space to produce a respective one of the symbol sub-streams.
 10. A transmitter adapted to transmit at least four symbol sub-streams, the transmitter comprising: a space-time encoder adapted to space-time code at least two pairs of symbol sub-streams, each of the pairs of symbol streams being space-time coded to form a respective pair of the transmit-sequence chains, the space-time coding being such that at least one of the formed pairs of the transmit-sequence chains is a function of symbols of the respective pair of symbol sub-streams and not a function of the symbols of the other pairs of the symbol sub-streams; and at least four transmit antennas, each having an input for receiving at least one of the at least four transmit-sequence chains, the input coupled to an output of the space-time encoder.
 11. The invention of claim 10, wherein each transmit sequence has a duration of four symbol periods; each transmit sequence of a particular transmit-sequence chains is a function of 1) a symbol of one of the symbol sub-streams of the respective symbol-sub-stream pair and 2) a complex conjugate of a symbol of the other symbol sub-stream of the respective symbol-sub-stream pair; and portions of the at least four transmit-sequence chains are representable by a matrix where: each row of the matrix represents one transmit sequence of a respective different one of the transmit-sequence chains, and each column of the matrix represents one symbol period.
 12. The invention of claim 11, wherein the matrix is orthogonal.
 13. The invention of claim 10, wherein portions of the at least four transmit-sequence chains are representable by a matrix where: each row of the matrix represents one transmit sequence of a respective different one of the transmit-sequence chains; each column of the matrix represents one symbol period; and the matrix is one of the matrices of the set of matrices consisting of: ${\begin{bmatrix} b_{1} & b_{1} & {- b_{2}^{*}} & {- b_{2}^{*}} \\ b_{2} & b_{2} & b_{1}^{*} & b_{1}^{*} \\ b_{3} & {- b_{3}} & {- b_{4}^{*}} & b_{4}^{*} \\ b_{4} & {- b_{4}} & b_{3}^{*} & {- b_{3}^{*}} \end{bmatrix}\quad {{and}\quad\begin{bmatrix} b_{1} & {- b_{2}^{*}} & 0 & 0 \\ b_{2} & b_{1}^{*} & 0 & 0 \\ 0 & 0 & b_{3} & {- b_{4}^{*}} \\ 0 & 0 & b_{4} & b_{3}^{*} \end{bmatrix}}},$

where: b₁ and b₂ are symbols of first and second symbol sub-streams, respectively, of one of the symbol-sub-stream pairs, b₃ and b₄ are symbols of first and second symbol sub-streams, respectively, of another of the symbol-sub-stream pairs, and b*₁, b*₂, b*₃, and b*₄ are complex conjugates of b₁, b₂, b₃, and b₄, respectively.
 14. The invention of claim 10, wherein the space-time encoder is adapted to spread at least a plurality of symbols of the transmit-sequence chains using a spreading code.
 15. The invention of claim 10, wherein the transmitter further comprises: an input; and at least one channel encoder being interposed between the input and the space-time encoder, the channel encoder adapted being to channel code a data sub-stream using a channel code.
 16. The invention of claim 15, wherein the transmitter further comprises at least one mapper, the mapper being interposed between the channel encoder and the space-time encoder, the mapper being adapted to map the channel coded data sub-stream into symbol-space to produce a respective one of the symbol sub-streams.
 17. A base station of a wireless communication system, the base station comprising the transmitter of claim
 10. 18. A mobile terminal comprising the transmitter of claim
 10. 19. The invention of claim 10, further comprising a plurality of radio frequency units, each having an input coupled to a respective output of the space-time encoder, each radio frequency unit adapted to convert a respective transmit sequence series from baseband to a radio frequency modulated signal.
 20. A receiver comprising: at least one receive antenna; and a matrix multiplier for multiplying a matrix with received symbol sub-streams of a signal received by the receive antenna, the matrix having at least two pairs of consecutive rows, each such pair being a function of channel characteristics of at least two channels that terminate on the receive antenna but not of channel characteristics of other channels that terminate on the receive antenna, and the matrix being orthogonal.
 21. The invention of claim 20, wherein the matrix is H†, which comprises one of the matrices of the set of matrices consisting of: ${\begin{bmatrix} h_{1}^{*} & h_{2}^{*} & h_{2} & h_{2} \\ h_{2}^{*} & h_{2}^{*} & {- h_{1}^{*}} & {- h_{1}} \\ h_{3}^{*} & {- h_{3}^{*}} & h_{4} & {- h_{4}} \\ h_{4}^{*} & {- h_{4}^{*}} & {- h_{3}} & h_{3} \end{bmatrix}\quad {{and}\quad\begin{bmatrix} h_{1}^{*} & h_{2} & 0 & 0 \\ {- h_{2}^{*}} & h_{1} & 0 & 0 \\ 0 & 0 & h_{3}^{*} & h_{4} \\ 0 & 0 & {- h_{4}^{*}} & h_{3} \end{bmatrix}}},$

where h₁, h₂, h₃, and h₄ are the complex channel characteristics of the channels between a 1^(st), 2^(nd), 3^(rd), and 4^(th) channel encoder, respectively and the receive antenna.
 22. The invention of claim 21, wherein the channels are flat-faded channels. 